This invention relates to a control system of the proportional plus integral type and particularly to a system wherein a single actuator serves to control power plant variables where the control responds to more than one parameter and the gain of the integrator is selected as a function of the error of the variable being controlled.
As is well known it is abundantly important that the gain of a control system is designed to match the response time of the system it is controlling. This is to assure quick response and stability in both the control and the system being controlled. Proportional plus integral types of controls are well known in the art and are utilized to optimize quick response and stable operation of the control dynamics by matching the gains to compensate for time lags and leads relative to the process dynamics. If the type of control responds to a single input and employs a single actuator, the dynamics of the system is relatively simple and stability and quick response are easily achieved by designing the system with the proper compensating time constants. However, a mismatch of response time in the process dynamics is apt to be present when a single actuator is controlled by more than one variable. For example, if the exhaust nozzle (A.sub.j) actuator for a gas turbine engine is controlled by an engine pressure ratio variable and an engine air flow variable, because of the time difference it requires for each variable to be affected by a change in area, it is necessary to design the control to compensate for this difference. Of course the design of multi-loop control system can be compromised but this is not ideal inasmuch as either speed and stability are compromised.
I have found that I can obviate this problem by designing the proportional plus integral control by changing the integral gain depending on which variable is controlling. Thus, this control scheme which included a single actuator being controlled by more than one variable has been engine tested and has proven to be particularly efficacious.